The present invention relates generally to inflatable occupant safety restraints in motor vehicles, and more particularly to reducing the toxicity of effluent gases produced by nonazide gas generating compositions.
Inflatable occupant restraint devices for motor vehicles have been under development worldwide for many years, including the development of gas generating compositions for inflating such occupant restraints. Because the inflating gases produced by the gas generants must meet strict toxicity requirements, most, if not all, gas generants now in use are based on alkali or alkaline earth metal azides, particularly sodium azide. When reacted with an oxidizing agent, sodium azide forms a relatively nontoxic gas consisting primarily of nitrogen.
However, azide-based gas generants are inherently volatile to handle and entail relatively high risk in manufacture and disposal. More specifically, whereas the inflating gases produced by azide-based gas generants are relatively nontoxic, the metal azides themselves are conversely highly toxic, thereby resulting in extra expense and risk in gas generant manufacture, storage, and disposal. In addition to direct contamination of the environment, metal azides also readily react with acids and heavy metals to form extremely sensitive compounds that may spontaneously ignite or detonate.
In contradistinction, nonazide gas generants, such as those disclosed in U.S. Pat. No. 5,139,588 to Poole, typically comprise a nonazide fuel selected from the group of tetrazole compounds and metal salts thereof, and provide significant advantages over azide-based gas generants with respect to toxicity related hazards during manufacture and disposal. Moreover, most nonazide gas generant compositions typically supply a higher yield of gas (moles of gas per gram of gas generant) than conventional azide-based occupant restraint gas generants.
However, nonazide gas generants heretofore known and used produce high levels of toxic substances upon combustion. The most difficult toxic gases to control are the various oxides of nitrogen (NO.sub.x) and carbon monoxide (CO).
Reduction of the level of toxic NO.sub.x and CO upon combustion of nonazide gas generants has proven to be a difficult problem. For instance, manipulation of the oxidizer/fuel ratio only reduces either the NO.sub.x or CO. More specifically, increasing the ratio of oxidizer to fuel minimizes the CO content upon combustion because the extra oxygen oxidizes the CO to carbon dioxide. Unfortunately, however, this approach results in increased amounts of NO.sub.x. Alternatively, if the oxidizer/fuel ratio is lowered to eliminate excess oxygen and reduce the amount of NO.sub.x produced, increased amounts of CO are produced.
One way to improve the toxicity of the combustion gases is to reduce the combustion temperature which would reduce the initial concentrations of both CO and NO.sub.x. Although simple in theory, it is difficult in practice to reduce the combustion temperature and to also retain a sufficiently high gas generant burn rate for practical application in an inflatable vehicle occupant restraint system. The burn rate of the gas generant is important to insure that the inflator will operate readily and properly. As a general rule, the burn rate of the gas generant decreases as the combustion temperature decreases. By using less energetic fuels, specifically fuels which produce less heat upon combustion, the combustion temperature may be reduced but the gas generant burn rate is also reduced.
Therefore, a need still exists for reducing the toxicity of effluent gases produced by nonazide gas generants without compromising the gas generant burn rate.